The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to purchase components for the base stations at a lower price and also wish to reduce the running costs of the base station. Active antenna arrangements with digital up-conversion have proven to meet these goals.
From EP 2 044 784 a digital transceiver is known that provides base-band to radio frequency up-conversion that operates in 4:3 operating mode (4:3 mode). The term “4:3 mode” in this context means that the sampling rate fsampling is 4/3 of the radio frequency carrier signal fCarrier.
A data signal is applied as a single bit stream to a digital analogue converter (DAC). The data signal may be input at rates of 2.5 Gbps. The incoming signal is first decoded, buffered, and de-multiplexed into an I-signal and a Q-signal by a deserialize decoder, frame buffer and de-multiplexer block. The de-multiplexed and separated I-signal and Q-signal are further transferred separately as 16-bit word length signals to over-sampling by sample-and-hold plus FIR low-pass filter units. The oversampled I-signal and the oversampled Q-signal are further passed from the over-sampling by sample-and-hold plus FIR low-pass filter units to systolic sigma-delta low-pass modulators. The I-signal and the Q-signal are applied to a quadrature modulation by multiplexing and cyclic negation block in which the I-signal and the Q-signal are over-sampled by 1:2, cyclically negated and 2:1 multiplexed to a single 2-bit output signal applied to a power digital to analogue converter.
The known digital transceiver from EP '784 receives the data signal from a remote radio unit via an interface according to the CPRI or OBSAI standard. The members of the Open Base Station Architecture Initiative (OBSAI) have specified a complete set of interface, hardware and test specifications covering the areas of transport, clock/control, radio and base band, together with hardware connection specifications. Similarly the goal of CPRI is to allow base stations manufacturers and component vendors to share a common protocol and more easily adapt platforms from one customer to another customer. Unlike the OBSAI standard, the CPRI standard does not specify mechanical or electrical interface requirements. However, both the OBSAI standard and the CPRI standard have in common that the data that base band information for the modulation of the radio signal is included in the serial data stream as in-phase (I) and quadrature (Q) samples.
The 4:3 mode compared to a 4:1 mode enables the use of lower clock frequencies for the delta-sigma modulator and a possible use of a switch mode power amplifier. However, as the output signal of the base band to radio frequency up converter in 4:3 mode is a return to opposite code, power amplifiers used in the transmitters have to be adapted to be compatible for the 4:3 mode.